For the purposes of biological treatment, compact biological reactors, which include anoxic (denitrification), aeration (nitrification) and sludge separation (clarifier) zones, and activated sludge recirculation system within one compact design are increasingly being used. Wastewater is treated by biological process using activated sludge, a mixture of microorganism, which requires ingredients contained in the wastewater for its growth and survival. During the process, ammonia compounds are oxidized to nitrite/nitrates (nitrification) and subsequently nitrite/nitrates are reduced to nitrogen gas (denitrification). In the separation zone, activated sludge is separated from the treated effluent by fluidized bed type filtration system and the separated activated sludge is recycled by means of airlift or mechanical pumps back to the anoxic zone.
The use of fluidized bed filtration-type processes for water treatment in such systems is also well known. Such processes generally rely on a decreasing upward velocity gradient that is formed by water flowing upwardly in a separator (or clarifier) through a fluidized bed created by agglomerated solids that are flowing downwardly to the bottom of the separator. Processes using this general concept are fully shown in described in U.S. Pat. Nos. 6,620,322, 5,755,966 and 5,720,876 and U.S. application, Publication No. US 2005/0000907 A1, the contents of which are herein incorporated by reference. The technical applications of such processes involve a wide range of chemical treatment of underground and surface water for communal and industrial use, and prevailing portion of municipal, industrial and agricultural biological wastewater treatment. Processes such as these rely on the flocculation of solids and a decreasing upward velocity profile through a “sludge blanket” formed by the flocculated solids. This sludge blanket, acting much like a fluidized bed, is responsible for the filtering of the generally upwardly flowing fluid. In order to get the desired decreasing upward velocity gradient, the separating space is usually formed with upwardly broadening diffuser shape cross sections. Common shapes that are used for this purpose include a simple inverted truncated cone, a longitudinal prism, a toroidal prism (which can also be described as an inverted truncated cone with inserted central cone or cylinder), among others.
The fluidized bed that is formed in the aforementioned process, generally called a sludge blanket, consists of a fluidized layer of flocculated solid waste particles. Inside the sludge blanket there is preferably formed a dynamic equilibrium: on one side the flocculation of smaller flocs leads to the creation of larger flocs and on the other side the larger flocs are disintegrated by hydrodynamic forces resulting from local turbulence thereto. The result of those two counteracting processes is a certain, generally uniform mean floc diameter and floc size distribution in a given place. In this manner, a fluidized bed like blanket may be formed having particles of generally standard size and shape.
Needless to say, in order to maintain dynamic equilibrium in a system utilizing a clarifier such as the one discussed above, there must be removal of suspended solids from the sludge blanket proportionally corresponding to the solids in the liquid flowing into the clarifier section. It is this removal that generally distinguishes the type of filtering that is being utilized. More specifically, in a fully fluidized bed system, the solids are generally withdrawn from the top of the sludge blanket. In a combined fluidized bed system, the solids are generally withdrawn from the middle of the sludge blanket. And in a partially fluidized bed system, the solids are generally withdrawn from the bottom of the sludge blanket. As will be discussed in detail below, the improved clarifier disclosed and claimed herein is generally useful in the partially or combined fluidized bed type systems.
In a partially fluidized bed system, the density current flows along the walls of the clarifier allowing the excess flocs to be removed at bottom of sludge blanket. Early on, this was accomplished using a simple return of separated suspended solids through the input. Later on, it was determined that increased performance could be obtained by forced withdrawal of the separated solids from below the propagated density currents. Because the concentration of flocs in density currents is higher than what is required for a full fluidization, the partially fluidized sludge blanket is particularly suitable for separation of concentrated suspensions such as may be found in various typical biological wastewater applications. An example of a clarifier using this method is disclosed and discussed in aforementioned U.S. Pat. No. 6,620,322.
Also advantageous is what may be described as a “combined” fluidized bed sludge blanket. In a combined sludge blanket system the bottom part of the sludge blanket behaves similar to a fully fluidized bed while the upper part behaves more like a partially fluidized bed. The fully fluidized bottom part distributes the water and solids into the upper partially fluidized part and the excess flocs are withdrawn from the density currents at the walls of the clarifier from the side at the middle part of the sludge blanket. Due to the fact that withdrawn density currents do not flow against liquid flow in the region of high apparent velocity, the hydraulic load can be substantially higher than in a pure partially fluidized sludge blanket system. As noted above, the partially fluidized and combined sludge blankets are particularly well suited to biological wastewater treatment facilities. As is well known, biological processes generally include systems for aerobic activation, aerobic sludge stabilization, nitrification, denitrificaton, dephosphorization and selector action.
In order to meet the requirements for effective activation process, the sludge blanket needs a significant concentration of activated sludge, generally obtainable only by sludge recirculation. More specifically, using an internal circulation loop, mixed liquor suspended solids (sludge) enter the clarifier from the aeration compartment at the bottom and are filtered out of the effluent by the filter media consisting of flocculated suspended solids themselves. The effectiveness of this process is critical to filtration efficiency. In fact, given that the portion of sludge that is “activated” represents only a small percentage of the total sludge in the sludge blanket, and given that the efficiency of the system depends on the effective activation of the sludge, the overall efficiency of the waste treatment system is very much dependent on the efficient recirculation and activation of the sludge at or near the bottom of the sludge blanket (in a partial fluidized bed system) or closer to the middle (in a combined fluidized bed system).
In order to insure most efficient activation of the sludge, great care must be taken to ensure that the sludge is removed from the bottom of the clarifier evenly across the length of the clarifier and that no ‘pockets’ of settled sludge are formed. This is because such pockets may lead to partial plugging and an uneven withdrawal of sludge from the clarifier, and these may cause anoxic conditions within the pocket, nitrogen gas generation due to denitrification and pockets of non-activated sludge rising to the surface of the clarifier, all of which detrimentally effect the overall efficiency of the treatment system. In doing this, care must also be taken to make sure that bubbles of oxygen from the aeration diffusers located in the oxidation zone are not accidentally introduced into the clarifier. In summary, it has been found that overall treatment efficiency can be detrimentally effected by recycle systems which do not allow for complete evacuation of the solids during “no flow” conditions.
Accordingly, it is desired to have a recycle system for use in a clarifier in a sludge blanket filtration system utilizing either partially fluidized or combined fluidized conditions that improves on prior designs with respect to the removal and recirculation of suspended solids, helps prevent the formation of settled sludge pockets, allows for almost complete evacuation of the solids during “no flow” conditions, and improves on other inefficient conditions inherent in treatment systems using prior art recycle designs.